It is known to prepare slub yarns by combining two yarns in a fluid texturing jet while periodically changing the feed rate of at least one of the feed yarns into the jet. It is also known to produce slubs at varied intervals in order to prevent an undesirable repeating pattern in fabric made from the slubbed product.
U.S. Pat. No. 3,091,909 discloses an apparatus for preparing a composite slub yarn by combining a core yarn and a second yarn in an air jet, the apparatus including a means for randomly actuating a device that changes the feed rate of the second yarn into the air jet so as to periodically feed an excess of the second yarn. In one embodiment this is accomplished by feeding the second yarn through a tension device which slows the speed of the yarn and then periodically releasing the tension device to give an increase in yarn speed. In a second embodiment, the actuating device controls a gear mechanism that changes the feed rate of the second yarn. Neither of the embodiments can control the speed of the second yarn rapidly enough to give short slubs when the yarns are fed at high rates of speed.
U.S. Pat. No. 3,805,344 describes an apparatus for preparing a composite slub yarn by combining an effect yarn and a core yarn in an air jet using a pivotally mounted rocker bracket to selectively press the effect yarn against different feed rolls which are rotating at different speeds. The rocker bracket is pivotally supported intermediate its ends and has first and second idler rolls mounted on opposite ends. By rotating the rocker bracket, the first and second idler rolls are made to alternately press the effect yarn against the high speed and low speed feed rolls, thus changing the feeding speed of the yarn. While this system gives very good control over the length of the slubs, there is a problem of filament breakage when the effect yarn contains fine denier filaments. This breakage is caused by the fact that when the idler roll is moved out of contact with the effect yarn its rotational speed slows substantially or even stops and then when the idler roll is moved back into contact with the rapidly moving effect yarn there is a substantial strain on the filaments as the idler roll is brought back up to speed.